The invention relates to gas injection systems, particularly of methane, for internal combustion engines, for the type comprising:
a plurality of electromagnetically controlled injectors, associated to the various cylinders in the engine,
a distributing manifold or rail communicating with said injectors,
a feeding reservoir of the distributing manifold where pressurised gas is accumulated,
a pressure regulating valve interposed in the connection between the reservoir and said distribution manifold,
an electronic control unit set up to control the injectors and to control the opening time to meter the amount of gas injected in each cylinder according to the operating conditions of the engine and,
an on-off valve arranged on the output of the reservoir.
The on-off solenoid valve complies with standards in force and ensures the possibility of hermetically closing the reservoir in any emergency condition, for example in the case of an accident entailing leakage of gas into the tubes located downstream to the reservoir.
The on-off valves used to date in systems of the type specified above use a deformable metallic material valve member, typically made of synthetic rubber, with consequent problems of tightness when cold. The use of plastic material valve members is consequently not advantageous, because there is the risk of the material forming the valve member being cut or sheared consequent to the pressure with which it is pressed against the corresponding valve seat, which again causes problems of tightness.
The object of the invention is to realise an on-off valve of the perfected type for use in gas injection systems of the type described above, which guarantees good tightness quality in all conditions of operation and particularly at all operating temperatures.
To obtain this object, the invention relates to a system of the type described in the preamble, characterised in that said on-off valve comprises a metallic material valve member, co-operating with a valve seat, also made of metallic material, and a solenoid to recall the valve member to an open condition against the action of elastic means.
The use of the metal-to-metal contact of the valve according to the invention ensures a fixed geometry of the valve in all operative conditions and, particularly, and at any operating temperature and pressure value.
In a practical embodiment of the invention, said valve member presents a ball conformation and the co-operating valve seat presents an impression defining an annular portion of spherical surface formed in a tapered portion of a gas outflow conduit.
According to an additional characteristic of the invention, the valve member is recalled to the closed condition by a spring tending to push it in the same direction as the flow of gas from reservoir.
The impression forming the valve seat can be obtained by coinage in the metallic side of said conduit for by machining with a hard metal rotary tool.
The valve geometry can be predetermined so that the valve also acts as a flow limiting valve for automatically cutting off the flow of gas if the delivery tends to increase over a certain value following a leak downstream to the reservoir, for example due to the breakage of a tube caused by an accident. For this purpose, the solenoid valve is suited to apply a force of attraction on the valve member which is higher in a first phase, in order to cause the detachment of the valve member from its seat, and lower in a second subsequent phase to keep the valve member open. During this phase, a flow of gas exceeding a predetermined threshold is thus capable of overcoming the relatively low attraction of the solenoid. Said result can be obtain by varying the power current of the solenoid and/or by suitably shaping the windings in the various areas of the solenoid.